(704) Interamnia, (779) Nina, (330825) 2008 XE3, and 2012 QG42 and Laboratory Study of Possible Analog Samples ⇑ Vladimir V

Icarus 262 (2015) 44–57

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Icarus

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Spectrophotometry of (32) Pomona, (145) Adeona, (704) Interamnia, (779) Nina, (330825) 2008 XE3, and 2012 QG42 and laboratory study of possible analog samples ⇑ Vladimir V. Busarev a,b, , Sergey I. Barabanov b, Vyacheslav S. Rusakov c, Vasiliy B. Puzin b, Valery V. Kravtsov a,d a Lomonosov Moscow State University, Sternberg Astronomical Institute, University Avenue 13, 119992 Moscow, Russia b Institute of Astronomy of Russian Academy of Science, Pyatnitskaya St. 48, 109017 Moscow, Russia c Division of Mossbauer Spectroscopy, Physical Dep. of Lomonosov Moscow State University, 119992 Moscow, Russia d Departamento de Fisica, Facultad de Ciencias Naturales, Universidad de Atacama, Copayapu 485, Copiapo, Chile article info abstract

Article history: Six asteroids including two NEAs, one of which is PHA, accessible for observation in September 2012 were Received 3 April 2015 investigated using a low-resolution (R 100) spectrophotometry in the range 0.35–0.90 lm with the aim Revised 3 July 2015 to study features of their reflectance spectra. A high-altitude position of our Terskol Observatory (3150 m Accepted 4 August 2015 above sea level) favorable for the near-UV and visible-range observations of celestial objects allowed us Available online 8 August 2015 to probably detect some new spectral features of the asteroids. Two subtle absorption bands centered at 0.53 and 0.74 lm were found in the reflectance spectra of S-type (32) Pomona and interpreted as signs of Keywords: presence of pyroxenes in the asteroid surface matter and its different oxidation. Very similar absorption Asteroids bands centered at 0.38, 0.44 and 0.67–0.71 lm have been registered in the reflectance spectra of (145) Spectrophotometry Mineralogy Adeona, (704) Interamnia, and (779) Nina of primitive types. We performed laboratory investigations of ground samples of known carbonaceous chondrites, Orguel (CI), Mighei (CM2), Murchison (CM2), Boriskino (CM2), and seven samples of low-iron Mg serpentines as possible analogs of the primitive asteroids. In the course of this work, we discovered an intense absorption band (up to 25%) centered at 0.44 lm in reflectance spectra of the low-Fe serpentine samples. As it turned out, the equivalent width of the band has a high correlation with content of Fe3+ (octahedral and tetrahedral) in the samples. It may be considered as a confirmation of the previously proposed mechanism of the absorption due to electronic transitions in exchange-coupled pairs (ECP) of Fe3+ neighboring cations. It means that the absorption feature can be used as an indicator of ferric iron in oxidized and hydrated low-Fe compounds on the surface of asteroids and other atmosphereless celestial bodies. Moreover, our measurements showed that the mechanism of light absorption is partially or completely blocked in the case of intermediate to high iron contents. Therefore, the method cannot probably be used for quantitative estimation of Fe3+ content on the bodies. Based on laboratory study of the analog samples, we conclude that spectral characteristics of Adeona, Interamnia, and Nina correspond to a mixture of CI–CM-chondrites and hydrated silicates, oxides and/or hydroxides. Spectral signs of sublimation activity on Adeona, Interamnia, and Nina at minimal heliocentric distances are likely discovered in the short-wavelength range (0.4–0.6 lm). It is suggested that such cometary-like activity at the highest surface temperatures is a frequent phenomenon for C and close type asteroids including considerable amounts of ices beneath the surface. A usual way of origin of a temporal coma of ice particles around a primitive asteroid is excavated fresh ice at recent impact event (s). The obtained reflectance spectra of two NEAs, (330825) 2008 XE3 and 2012 QG42, are predominantly featureless and could be attributed to S(C) and S(B)-type bodies, respectively. We discuss reasons why weak spectral features seen in reflectance spectra of the main-belt asteroids are not observed in those of NEAs. Ó 2015 Elsevier Inc. All rights reserved.

⇑ Corresponding author at: Lomonosov Moscow State University, Sternberg Astronomical Institute, University Avenue 13, 119992 Moscow, Russia. E-mail address: [email protected] (V.V. Busarev). http://dx.doi.org/10.1016/j.icarus.2015.08.001 0019-1035/Ó 2015 Elsevier Inc. All rights reserved. V.V. Busarev et al. / Icarus 262 (2015) 44–57 45

1. Introduction calibration of the spectra was done using the positions of hydrogen Balmer lines in the spectrum of a Peg (B9III) observed in a repeated Spectrophotometry/spectroscopy is a traditional method of mode. The total exposure time spent on each target was typically h remote study of asteroids and other atmosphereless celestial bod- 1–2 . The obtained reflectance spectra were corrected for the dif- ies (e.g., McCord et al., 1970; Adams, 1974). When ground-based ference in air mass by applying a conventional method based on telescopes used, the range preliminary from 0.38 to 1.1 lmis using observations of a solar analog star (e.g., McCord et al., extended up to 2.5 lm (e.g., Vernazza et al., 2008; DeMeo et al., 1970). In our work a single solar analog star, HD 10307 (G1.5V) 2009; Hardersen et al., 2014; Fieber-Beyer et al., 2015). It is defined (Hardorp, 1980), was intentionally exploited to avoid possible dif- by the boundaries of the most transparent spectral ‘‘window” of ferences in the calculated reflectance spectra of asteroids as in the the Earth’s atmosphere, through which the bulk of observational case of several solar analogs use. Observations of the same star information on asteroids was obtained. It allowed us to enrich were performed to determine the running spectral extinction func- our knowledge about these objects, in particular on their taxonomy tion of the terrestrial atmosphere (Busarev, 2011). The observa- (e.g., Tholen, 1989; Bus and Binzel, 2002a, 2002b; DeMeo et al., tions of HD 10307 were made nearly in the same range of the air 2009). Further progress in ground-based studies of asteroids are masses at (or close to) which the asteroids of the sample were naturally related with increasing both the number (and therefore observed (see Table 1). The values of the signal-to-noise ratio the sample size) of studied bodies and the accuracy of spectral (S/N) of the asteroid spectra were estimated in the range of measurements. High-quality reflectance spectra of asteroids 0.4–0.8 lm. They are given, along with other data, in Table 1.To potentially contain not only valuable mineralogical information reduce high-frequency fluctuations in the reflectance spectra, they on the material of which the asteroids are made but also that on were smoothed by the method of ‘‘running box average” with a the valency state of iron (as well as of other transition metals). 5-point averaging interval. This allowed us to study considerably As is known, the latter depends on physico-chemical parameters wider spectral features of the observed asteroids and to assess (Platonov, 1976; Burns, 1993) of asteroid matter connected with their spectral types according to shapes of their reflectance spectra. the formation conditions of the bodies and their subsequent evolu- As a rule, averaging of the asteroid consecutive reflectance spectra tion. Unfortunately, various distorting factors, such as observa- was made when they had a close overall shape and observational tional faults and space weathering, make it difficult to spectra were obtained at minimal air masses. reconstruct reliably the previous conditions. Thus, the final goal Ephemerides (taken from the IAU Minor Planet Center on-line of such kind of study is to accurately extract the observational service at http://www.minorplanetcenter.net/iau/MPEph/MPEph. information and to try to correctly interpret it. For most asteroids, html) and observation parameters of the asteroids are given in except for some bodies investigated by space methods, there is a Table 1. lack of data about whether or not chemico-mineralogical and other properties vary along their surface. The reason of that is mainly due to (nearly) point-like appearance of asteroids at ground- 3. Analysis and interpretation of asteroid reflectance spectra based observations, which makes it difficult to obtain spectral information of different parts of the asteroids’ surface. Indeed, 3.1. 32 Pomona the angular size of (1) Ceres, the largest asteroid with a diameter of 1000 km, varies in the range 0.008–0.003, which is comparable Average diameter and geometric albedo of Pomona according to to the limiting angular resolution of ground-based telescopes at recent WISE-data are of 81.78 km and 0.25 (Masiero et al., 2014). h the excellent atmospheric seeing. The most important is to mini- The asteroid rotates with a period of 9.448 (Harris et al., 2012). mize the impact of the Earth’s atmosphere on reflectance spectra In total, eight separate spectra of Pomona were registered on the of asteroids and on the reliability of final results and conclusions. night 19/20 of September, 2012, together with the spectra of To achieve this goal it is useful to compare spectral data obtained HD10307 used as a solar analog star (Table 1). We used them to with the same facility on asteroids of the same and/or close taxo- calculate an average spectrum of the asteroid and normalized it nomic types which are expected to have similar spectral features. to 1.0 at 0.55 lm(Fig. 1a). It corresponds to an S-type body having Such approach is used in the work to study several of C-B-type mineralogy dominated by pyroxenes, olivines, and other high- asteroids. As before, a laboratory study of spectral characteristics temperature compounds (e.g., Gaffey et al., 1989, 1993). Given of both meteoritic and terrestrial analog samples is very helpful the rotational period of the asteroid, the total exposure time spent and used also in our work. In addition, we consider and discuss to obtain the eight spectra (2 h) corresponds to 1/5 of its rota- the reflectance spectra obtained by other authors for the objects tional period. In terms of its shape, the spectrum is very similar of our sample to the date. to that obtained by McCord and Chapman (1975a) but differs to some extent from that by Bus and Binzel (2002a or 2003a) and Xu et al. (1995) (Fig. 1b). We find two relatively weak and broad 2. Observations and data reduction absorption bands in the obtained spectra of Pomona at 0.49–0.55 lm and 0.73–0.77 lm(Fig. 1a). We suppose that the for- Spectrophotometry of the Asteroids (32) Pomona, (145) Adeona, mer one, at 0.49–0.55 lm, has a complex nature. It is probably a (704) Interamnia, (779) Nina, (330825) 2008 XE3, and 2012 QG42 superposition of two or even three wide spectral features. It was was performed in September 2012 using the 2-m telescope of established earlier that electronic spin-forbidden crystal-field tran- Terskol Observatory operated by Institute of Astronomy of Russian sitions in Fe2+ ions in M2 crystallographic positions of clinopyrox- Academy of Science (IA RAS). The observatory is situated at high enes are responsible for a couple of weak bands near 0.505 lm and altitude of 3150-m above sea level, making especially favorable 0.550 lm (e.g., Burns et al., 1973; Platonov, 1976; Hazen et al., conditions for observations at shorter wavelengths. The telescope 1978; Matsyuk et al., 1985; Burns, 1993; Klima et al., 2006). If is equipped with a prism CCD-spectrometer (WI CCD asteroid surface matter consists of a clinopyroxene and Fe orthopy- 1240 1150 pix.) working in the range 0.35–0.97 lm, with roxene mixture then an additional weak band can originate at R 100 resolving power. DECH spectral package (Galazutdinov, 0.525 lm due to the spin-forbidden crystal-field electronic 1992) was employed to reduce CCD observations by means of stan- d–d-transitions in Fe2+ ions in M1 crystallographic sites of Fe dard reduction procedures (such as flat-field correction and bias orthopyroxenes. Moreover, a common absorption band at and dark subtraction) and to extract asteroid spectra. Wavelength 0.49–0.55 lm (as any other one) could be widened because of 46 V.V. Busarev et al. / Icarus 262 (2015) 44–57

Table 1 Ephemerides and observational parameters of asteroids for moments of all obtained spectra.

Date (y m d) UT meadle (hms) R.A. (h m s) Decl. (°0) Delta (AU) r (AU) Elong. (°) Ph. (°) V(m) Elev. (°) Exp. time (s) Air mass S/N 32 Pomona 2012 09 19 234100 02 01.01 +15 10.0 1.926 2.799 143.7 12.3 12.0 61.2 1200 1.1434 9.3 2012 09 20 000200 02 01.01 +15 09.9 1.925 2.799 143.8 12.2 12.0 60 1200 1.1543 7.2 2012 09 20 002300 02 01.00 +15 09.9 1.925 2.799 143.8 12.2 12.0 58 1200 1.1788 6.2 2012 09 20 004300 02 00.99 +15 09.8 1.925 2.799 143.8 12.2 12.0 57 1200 1.2062 10 2012 09 20 005400 02 00.99 +15 09.8 1.925 2.799 143.8 12.2 12.0 55 600 1.2202 7.6 2012 09 20 010500 02 00.98 +15 09.8 1.925 2.799 143.8 12.2 12.0 53 600 1.2515 7.5 2012 09 20 011600 02 00.98 +15 09.7 1.925 2.799 143.8 12.2 12.0 51.5 600 1.2770 6.3 2012 09 20 012700 02 00.98 +15 09.7 1.925 2.799 143.8 12.2 12.0 50 600 1.3046 6 Solar analog star HD10307: Elev. 61° t =01h 30m, air mass 1.1430 145 Adeona 2012 09 19 225400 02 29.58 01 38.1 1.843 2.692 140.1 13.9 12.4 43.5 1200 1.4512 3 2012 09 19 231500 02 29.57 01 38.1 1.843 2.692 140.1 13.9 12.4 44.5 1200 1.4253 3.2 Solar analog star HD10307: Elev. 67° t =01h 32m, air mass 1.0862 704 Interamnia 2012 09 13 211100 03 24.64 +39 00.2 2.080 2.616 111.1 21.0 10.9 47 600 1.3662 5 2012 09 13 212200 03 24.64 +39 00.2 2.080 2.616 111.1 21.0 10.9 49 600 1.3242 5.1 2012 09 13 213300 03 24.64 +39 00.3 2.080 2.616 111.1 21.0 10.9 51 600 1.2860 4.9 2012 09 13 214400 03 24.65 +39 00.3 2.080 2.616 111.1 21.0 10.9 53 600 1.2515 5 2012 09 13 215500 03 24.65 +39 00.4 2.080 2.616 111.1 21.0 10.9 55 600 1.2202 5 Solar analog star HD10307: Elev. 48° at t =19h 25m, air mass 1.3446 779 Nina 2012 09 13 175100 00 53.11 +32 40.7 1.289 2.149 138.5 18.1 11.0 35.5 300 1.7190 4.4 2012 09 13 175700 00 53.11 +32 40.7 1.289 2.149 138.5 18.1 11.0 36.5 300 1.6783 4.5 2012 09 13 180300 00 53.10 +32 40.7 1.289 2.149 138.5 18.1 11.0 37.5 300 1.6401 4.6 2012 09 13 180900 00 53.10 +32 40.7 1.289 2.149 138.5 18.1 11.0 38.5 300 1.6040 4.6 2012 09 13 181500 00 53.10 +32 40.7 1.289 2.149 138.5 18.1 11.0 39.5 300 1.5700 5.5 2012 09 13 201300 00 53.04 +32 41.0 1.289 2.149 138.6 18.0 11.0 61.5 300 1.1376 5.5 2012 09 13 201900 00 53.04 +32 41.0 1.289 2.149 138.6 18.0 11.0 62.5 300 1.1271 5.3 2012 09 13 202500 00 53.03 +32 41.0 1.289 2.149 138.6 18.0 11.0 63.5 300 1.1171 5.4 2012 09 13 203100 00 53.03 +32 41.1 1.288 2.149 138.6 18.0 11.0 64.5 300 1.1077 5.5 2012 09 13 203700 00 53.03 +32 41.1 1.288 2.149 138.6 18.0 11.0 65.5 300 1.0987 4.3 2012 09 13 224400 00 52.97 +32 41.4 1.288 2.149 138.7 18.0 11.0 79 900 1.0187 5.3 2012 09 13 230000 00 52.96 +32 41.4 1.288 2.149 138.7 18.0 11.0 78 900 1.0223 5.1 Solar analog star HD10307: Elev. 48° at t =19h 25m, air mass 1.3446 (330825) 2008 XE3 2012 09 12 223200 01 37 07.8 +37 18.0 0.246 1.176 128.6 42.0 15.2 79.5 1800 1.0170 2 2012 09 12 230300 01 37 11.7 +37 18.9 0.246 1.176 128.6 42.0 15.2 83 1800 1.0075 2 2012 09 12 233400 01 37 15.7 +37 19.9 0.246 1.176 128.6 42.0 15.2 83 1800 1.0075 3.2 2012 09 13 003600 01 37 23.5 +37 21.7 0.246 1.176 128.6 42.0 15.2 74 1800 1.0402 2.8 2012 09 13 010700 01 37 27.4 +37 22.6 0.246 1.176 128.6 42.0 15.2 68.5 1800 1.0746 4.9 Solar analog star HD10307: Elev. 67° at t =01h 32m, air mass 1.0862 2012 QG42 2012 09 12 190700 20 35 40.4 +10 22.8 0.021 1.021 135.7 43.5 14.2 55 1200 1.2202 9 2012 09 12 193200 20 34 38.3 +10 28.7 0.021 1.021 135.4 43.7 14.3 53 1200 1.2515 7.4 2012 09 12 195300 20 33 46.0 +10 33.6 0.021 1.021 135.2 44.0 14.3 51 1200 1.2860 6.4 2012 09 12 201400 20 32 53.4 +10 38.6 0.021 1.021 135.0 44.2 14.3 48.5 1200 1.3342 6.8 Solar analog star HD10307: Elev. 67° at t =01h 32m, air mass 1.0862

Description of designations: UT – universal time; R.A. – right ascension; Decl. – declination; Delta – distance from the Earth’s center to the object’s center; r – distance from the object’s center to the Sun’s center; Elong. – elongation of the object; Ph. – phase angle of the object; V – visual stellar magnitude of the object; Elev. – elevation of the object above the local horizon; Exp. time – exposure time; Air mass – atmospheric air mass of the observed object; S/N – signal to noise ratio for the object.

disordering crystal structure at the process of space weathering of these mechanisms could strengthen the discussed Pomona’s the asteroid surface (e.g., Charette et al., 1974; Gaffey et al., 1993). absorption band at 0.49–0.55 lm. This assumption seems to be On the other hand, as shown in investigations of terrestrial analog/ supported by the presence of additional absorption band in the meteoritic samples (Rossman, 1975; Bakhtin, 1985; Matsyuk et al., reflectance spectra of Pomona at 0.73–0.77 lm(Fig. 1a). It could 6 4 3+ 1985; Khomenko and Platonov, 1987; Taran and Rossman, 2002), originate due to electronic A1 ? T1 transitions in Fe ions and some intense absorption bands could arise at the same positions located on the short-wavelength wing of the intense Fe2+ absorp- as spin-forbidden crystal-field electronic d–d-transitions in Fe2+ tion band (spin-allowed) of pyroxene–olivine mixture centered at and Fe3+ ions. Probable mechanisms of such intensification are 0.9–1.0 lm(Fig. 1a). Its parameters are similar to those of absorp- additional electronic transitions in magnetic exchange-coupled tion bands registered in spectra of Fe3+-bearing terrestrial and pairs (ECP) of neighboring cations, Fe2+–Fe3+ (Mattson and lunar pyroxenes (e.g., Mattson and Rossman, 1984; Bakhtin, Rossman, 1984; Matsyuk et al., 1985)orFe3+–Fe3+ (Rossman, 1985; Matsyuk et al., 1985; Khomenko and Platonov, 1987; 1975; Sherman, 1985; Sherman and Waite, 1985; Taran and Straub and Burns, 1990; Burns, 1993). Thus, the observed spectral Rossman, 2002) having common ligands. We do not exclude that features of Pomona are presumably the manifestations of V.V. Busarev et al. / Icarus 262 (2015) 44–57 47

Fig. 2. (a) Averaged and normalized at 0.55 lm reflectance spectrum of (145) Adeona. (b) Comparison of the same spectrum (1) with Adeona’s spectra obtained in SMASSII (Bus and Binzel, 2003b), in 52-color photometric survey (Bell et al., 1995), and by McCord and Chapman (1975b) (inset ‘‘A”). Fig. 1. (a) Averaged and normalized at 0.55 lm reflectance spectrum of (32) Pomona. (b) Comparison of the same spectrum (1) with Pomona’s spectra obtained in SMASSII (Bus and Binzel, 2003a) (2), in SMASS (Xu et al., 1995) (3), and by Table 2), though it is comparable to the spectral noise in the range. McCord and Chapman (1975a) (inset ‘‘A”). Such a band might also be formed due to electronic ECP-transitions 3+ 3+ 6 6 4 4 in Fe –Fe pairs ( A1( S) ? E( D)) (Sherman, 1985; Bakhtin, 1985; Khomenko and Platonov, 1987). We think that the detection predominant pyroxene–olivine mixture (e.g., Singer, 1981) and its of the two latter bands was possible thank to a high-altitude loca- various oxidations (e.g., Straub and Burns, 1990) in the surface tion of our observatory favorable for observations in the near-UV. matter. We compared our spectra of Adeona with those available in the literature (Fig. 2b). Actually, the presence of absorption bands at 0.38 3.2. (145) Adeona, (704) Interamnia, and (779) Nina and 0.44 lm in reflectance spectra of the asteroid were not previously reported. This is likely because the published spectra have (145) Adeona is a 126.13-km asteroid with geometric albedo of different wavelength ranges, in particular less extended to the 0.06 (averaged WISE-data from Masiero et al., 2014) rotating with blue. A weak 0.43-lm absorption band was detected in reflectance a period of 15.071h (Harris et al., 2012). Its Tholen and SMASSII spectra of other eleven asteroids of primitive C, P, G classes by Vilas spectral types (Tholen, 1989; Bus and Binzel, 2002b) are C and et al. (1993). A notable absorption band (10%) centered near Ch (‘‘h” means that the asteroid is hydrated because of the pres- 0.68–0.70 lm presents in reflectance spectra of Adeona according ence of a 0.7 lm absorption band, according to Bus and Binzel, to McCord and Chapman (1975b) (in Fig. 2b, inset ‘‘A”) and 2002b), respectively. The resulting averaged and normalized (at Fornasier et al. (2014). Similar absorption features were found in 0.55 lm) reflectance spectrum of Adeona is shown in Fig. 2a. We reflectance spectra of many other asteroids (Vilas and Gaffey, have found three absorption bands in the spectrum. The strongest 1989; Vilas et al., 1994; Bus and Binzel, 2002a; Fornasier et al., one is located between 0.48 and 0.85 lm. Judging by its parame- 2014). It is attributed to Fe2+ ? Fe3+ IVCT-transitions in hydrated ters (Table 2) after removing the continuum slope (Fig. 5), the band silicates (Vilas and Gaffey, 1989). Bus and Binzel (2002b) used probably arises due to electronic intervalence charge transfer the band in their classification system to delineate C and Ch (as (IVCT) transitions Fe2+ ? Fe3+ in hydrated silicates, as follows from well Cg and Cgh) type asteroids. However, we see that 0.7-lm investigations of terrestrial samples (e.g., Platonov, 1976; Bakhtin, absorption band in our reflectance spectrum of Adeona is several 1985; Burns, 1993). The second one, revealed itself between 0.40 times more intense then that in data of McCord and Chapman and 0.47 lm(Fig. 2a), is probably due to electronic ECP- (1975b) and Fornasier et al. (2014). There is also an unexplainable 3+ 3+ 6 6 4 4 l transitions in exchangeable Fe –Fe pairs ( A1( S) ? T2( G)) in sharp rise of reflectivity in interval of 0.35–0.55 m in our data of oxides, hydrated oxides and hydrated silicates (Sherman, 1985; Adeona (Fig. 2b). We thoroughly checked our observational data Sherman and Waite, 1985; Bakhtin, 1985; Khomenko and but did not find any faults. We suppose that the difference is real Platonov, 1987). The presence of a third band with relative inten- and should be explained. We will try to do this by the end of the sity lower than 5% is suspected at 0.36–0.39 lm (Figs. 2a and 7, section. 48 V.V. Busarev et al. / Icarus 262 (2015) 44–57

Table 2 Parameters of absorption bands detected in reﬂectance spectra of observed asteroids, carbonaceous chondrites and terrestrial serpentines.

Object Spectral parameters Band center Band width Band relative Interpretation References (lm) (lm) intensity (%) Asteroids (32) Pomona 0.53 0.49–0.56 5–7 Spin-forbidden d–d-transitions in Fe2+ strengthened by [1–2, 7–9] ECP-transitions in Fe2+–Fe3+ pairs 0.74 0.69–0.77 3–5 Spin-forbidden d–d-transitions in Fe3+ strengthened by [1–2, 7–9] ECP-transitions in Fe3+–Fe3+ pairs 0.90–1.0 0.68–? 27 Spin-allowed d–d-transitions in Fe2+ [3, 6–8] (145) Adeona 0.38 0.37–0.39 8? ECP-transitions in Fe3+–Fe3+ pairs [4–9] 0.44 0.41–0.47 10? ECP-transitions in Fe3+–Fe3+ pairs [4–9] 0.67 0.49–0.85 32? IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] (704) Interamnia 0.38 0.36–0.40 20? ECP-transitions in Fe3+–Fe3+ pairs [4–9] 0.46 0.43–0.48 11? ECP-transitions in Fe3+–Fe3+ pairs [4–9] 0.70 0.55–0.85 24? IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] (779) Nina 0.39 0.36–0.41 25? ECP-transitions in Fe3+–Fe3+ pairs [4–9] 0.46 0.42–0.49 19? ECP-transitions in Fe3+–Fe3+ pairs [4–9] 0.71 0.55–0.85 32? IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] (330825) 2008 XE3 – – – 2012 QG42 1.0? 0.50–? 50 Spin-allowed d–d-transitions in Fe2+ [3, 6–8] and/or IVCT-transitions Fe2+ ? Fe3+ Carbonaceous chondrites Orguel (CI) 0.49 0.46–0.54 5 Spin-forbidden d–d-transitions in Fe2+ strengthened [1–2, 7–9] by ECP-transitions in Fe2+–Fe3+ pairs 1.0 0.6–? 10 IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] Mighei (CM2) 0.80 0.58–1.0 5 IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] Murchison (CM2) 0.75 0.59–0.82 12 IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] 0.90 0.82–1.0 10 IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] Boriskino (CM2) 0.90 0.54–? 22 IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] Terrestrial serpentines 1a 0.365 0.360–0.370 1 Spin-forbidden d–d-transitions in Fe3+ [1–2, 7–9] 0.390 0.380–0.400 2 Spin-forbidden d–d-transitions in Fe3+ [1–2, 7–9] 0.450 0.408–0.485 23 ECP-transitions in Fe3+–Fe3+ pairs [4–9] 0.750 0.555–1.0 20 IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] 4a 0.390 0.370–0.400 1 Spin-forbidden d–d-transitions in Fe3+ [1–2, 7–9] 0.450 0.408–0.480 6 ECP-transitions in Fe3+–Fe3+ pairs [4–9] 4b 0.450 0.408–0.485 7 ECP-transitions in Fe3+–Fe3+ pairs [4–9] 10a 0.390 0.370–0.400 1 Spin-forbidden d–d-transitions in Fe3+ [1–2, 7–9] 0.443 0.400–0.485 8 ECP-transitions in Fe3+–Fe3+ pairs [4–9] 0.750 0.690–0.790 3 IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] 28 0.365 0.360–0.370 1 Spin-forbidden d–d-transitions in Fe3+ [1–2, 7–9] 0.390 0.380–0.400 2 Spin-forbidden d–d-transitions in Fe3+ [1–2, 7–9] 0.445 0.408–0.482 25 ECP-transitions in Fe3+–Fe3+ pairs [4–9] 30 0.445 0.400–0.480 Signs ECP-transitions in Fe3+–Fe3+ pairs [4–9] 0.860 0.650–? 20 IVCT-transitions Fe2+ ? Fe3+ [3, 6–8] 2540 0.365 0.360–0.370 1 Spin-forbidden d–d-transitions in Fe3+ [1–2, 7–9] 0.390 0.380–0.400 3 Spin-forbidden d–d-transitions in Fe3+ [1–2, 7–9] 0.445 0.408–0.480 13 ECP-transitions in Fe3+–Fe3+ pairs [4–9]

References: [1] Robbins and Strens (1972); [2] Rossman (1975); [3] Platonov (1976); [4] Sherman (1985); [5] Sherman and Waite (1985); [6] Matsyuk et al. (1985); [7] Khomenko and Platonov (1987); [8] Burns (1993); [9] Taran and Rossman (2002).

Average diameter and geometric albedo of (704) Interamnia are spectrum (from several) with the highest S/N ratio. After the con- 307.31 km and 0.08 according to WISE-data (Masiero et al., 2014). tinuum slope removal, the absorption spectral features and their It rotates with a period of 8.727h (Harris et al., 2012). The asteroid parameters are presented in Figs. 5–7 and Table 2. Again, we find was previously classified as F-type body by Tholen (1989) and as that the majority of Interamnia’s spectra available in the literature B-type one by Bus and Binzel (2002b) characterizing by a wide are less extended to the blue than ours (in particular, Bus and absorption band beyond 0.5 lm coupled with an overall negative Binzel, 2002a; Lazzaro et al., 2004; Fornasier et al., 2014) except gradient. In total, five spectra of Interamnia were obtained for for that by Chapman et al. (1973) obtained in a wider spectral 3/4h corresponding to a change of its relative rotation phase range. We compare some of them with ours in Fig. 3b. As in the 0.08 (Table 1). We averaged these similar in shape spectra and case of Adeona, our reflectance spectrum of Interamnia demon- showed as one resulting normalized (at 0.55 lm) spectrum in strates a sharp rise of reflectivity at 0.4–0.7 lm clearly seen on a Fig. 3a. It appears similar to that of Adeona, with virtually the same wider scale (Fig. 3a). We will return to this point in general, after shape and relative intensity of features centered at approximately consideration data on the next asteroid. 0.38, 0.46, and 0.70 lm (see Figs. 2a and 3a). Therefore, the same (779) Nina is a 80.57 km asteroid with geometric albedo of 0.16 interpretation to them should be given. It should be also taken into according to WISE-data (Masiero et al., 2014) rotating with a per- account that slightly different averaging methods have been used iod of 11.186h (Harris et al., 2012). It was previously classified as for calculation of the mentioned reflectance spectra of the asteroid. M-type asteroid by Tholen (1989) and as X-type one according to While we and Fornasier et al. (2014) used the method of simple SMASSII taxonomy (Bus and Binzel, 2002b). In total, twelve obser- averaging, Bus and Binzel (2002a) applied the method of weighted vational spectra of Nina were obtained (Table 1). During the obser- averaging. In turn, Lazzaro et al. (2004) left a single asteroid vations, Nina has rotated by 0.5 of its period. One can see that, in V.V. Busarev et al. / Icarus 262 (2015) 44–57 49

Fig. 5. Normalized absorption bands centered at 0.50–0.71 lm in the reﬂectance spectra of (145) Adeona, (704) Interamnia, and (779) Nina after approximation and removal of the continuum.

Fig. 3. (a) Averaged and normalized at 0.55 lm reﬂectance spectrum of (704) Interamnia. (b) Comparison of the same spectrum (1) with Adeona’s spectra obtained in SMASS (Lazzaro et al., 2006), in SMASSII (Bus and Binzel, 2003c), and in 52-color photometric survey (Bell et al., 1995).

Fig. 6. Normalized absorption bands centered at 0.44–0.46 lm in the reﬂectance spectra of (145) Adeona, (704) Interamnia, and (779) Nina along with serpentine samples (numbered as 1a, 4a, 4b, 10a, 28, and 2540) after approximation and removal of the continuum.

Fig. 7. Normalized absorption bands centered at 0.38–0.39 lm in the reﬂectance spectra of (145) Adeona, (704) Interamnia, and (779) Nina after approximation and removal of the continuum.

fact, the presented reflectance spectrum of Nina (Fig. 4a) is similar to that of Interamnia in (Fig. 3a). There are virtually the same absorption bands in the spectra of Nina (located approximately at 0.39, 0.46 and 0.71 lm), Adeona, and Interamnia (see Figs. 4a and 5–7). Fig. 4. (a) Averaged and normalized at 0.55 lm reflectance spectrum of (779) Nina. After the continuum slope removal, the absorption spectral fea- (b) Comparison of the same spectrum (1) with Nina’s spectra obtained in SMASS (Lazzaro et al., 2006), in SMASSII (Bus and Binzel, 2003d), and with IRTF tures of Nina and their parameters are presented in Figs. 5–7 and (Ockert-Bell, 2011). Table 2. Some differences of Nina’s absorption features are their 50 V.V. Busarev et al. / Icarus 262 (2015) 44–57 somewhat larger width and slightly longer wavelength positions as atmosphereless solid body taking into account the Stefan–Boltz- compared to those of Adeona and Interamnia. Therefore, the origin mann constant r = 5.6704 108 Js1 m2 T4 (or in W m2 T4) of these absorption bands in the spectra of Nina and of their and re = 1 AU: respective counterparts in the spectra of Adeona and Interamnia = ¼ðð Þ p 2= p r 2Þ1 4 are consistent. It should be mentioned that there are differences Tss 1 pm Ce 4 re 4 r = between the average geometric albedo of Adeona (0.043), Interam- 2 2 1 4 ¼ðð1 pmÞC r =r r Þ nia (0.08), and Nina (0.16) related probably to differences in the e e = 8 4 2 1 4 predominant content of their surface matter. Yet, radar albedo of ¼ðð1 pmÞ241:08 10 K =r Þ or Nina is typical for a primitive type asteroid (Shepard et al., 2010). 1=4 T ¼ 394 K ðð1 p Þ=r2Þ ð1Þ The presence of the spectral features at 0.38 and 0.44 lm cannot ss m be checked in spectra of Nina, obtained by Bus and Binzel Calculations by the formula give next subsolar temperatures on (2002a) and Lazzaro et al. (2004), because they are less extended the asteroids at the moments of observations: Tss,o = 236.5 K to the blue as compared with ours. There are signs of one feature (Adeona), Tss,o = 238.6 K (Interamnia), and Tss,o = 257.3 K (Nina). centered at 0.6 lm in the asteroid spectrum obtained in SMASS As it turns out, the Adeona’s and Interamnia’s temperatures are by Lazzaro et al. (2004) (Fig. 4b). Again, as seen from our reflec- very similar. All the temperature values are in the temperature tance spectrum of Nina presented along with IRTF and other data range of the most intense sublimation of water ice: within several in the range up to 2.5 lm(Fig. 4b), there is a considerable growth tens of degrees below 273 K. of reflectivity of the asteroid between 0.4 and 0.7 lm. Our consideration of Adeona’s, Interamnia’s, and Nina’s reflec- Thus, a similar unexplainable effect found in reflectance spectra tance spectra (Figs. 2–4), their interpretation and comparison with of Adeona, Interamnia, and Nina, three asteroids of primitive types, those of possible analog samples (Figs. 10 and 11) show that scat- observed by us within a week (Table 1). We noted that Interamnia, tering of reflected light in hypothetical asteroid coma of sublimed and Nina in the time of observations were near their perihelion dis- ice particles does not change position of intrinsic asteroid absorp- tances, when temperature on the asteroid surface reached a max- tion bands. However, it may change their intensity, especially for imum near the subsolar point. At the same time, approaching to 0.7-lm band which minimum almost coincides with the change the Sun Adeona was close to its middle heliocentric distance. We of continuum gradient sign. For this reason, we have queried all suspected that the registered unusual short-wavelength raise of the estimated values of relative intensity of the found absorption asteroid reflectance may be result of a sublimation activity of the features (Table 2). bodies’ surface matter including ices. A cloud or coma of sublimed and frozen particles surrounding an asteroid could produce a con- 3.3. (330825) 2008 XE3 and 2012 QG42 siderable scattering of light reflected from the asteroid. Given spectrally neutral refractive index of the particles in the used range, the Two NEAs were observed by us, as well. (330825) 2008 XE3, a wavelength position of a ‘‘hump” created by scattering of reflected light in asteroid reflectance spectrum is likely determined by pre- member of Amor group, and 2012 QG42, PHA and a member of Apollo group (http://ssd.jpl.nasa.gov/sbdb.cgi#top), have rota- dominant particle sizes in the coma. As seen in Figs. 2a, 3a, and 4a, h h it takes place around 0.4–0.6 lm. tional periods of 4.409 (Warner, 2013) and 24.22 (Warner et al., 2013), respectively. The obtained spectra of 330825 were The surface temperatures of main-belt asteroids are estimated by direct measurements and modeling. For instance, the Rosetta/ registered for about 2.5 h that corresponds to change in its rotational phase by 210°. VIRTIS measurements showed that the surface temperature on l (21) Lutetia varies between 170 and 245 K (Coradini et al., 2011). Normalized at 0.55 m reflectance spectra of the asteroid are shown in Fig. 8. Spectrum 2 is arbitrarily displaced relative to spec- Among volatiles abundant near planetary surfaces (e.g., Dodson- Robison et al., 2009), the triple point in the phase diagram for trum 1 to facilitate their comparison. Spectra 1 and 2 are the averages of three and two consecutive reflectance spectra, respectively. H2O and CO2 (and their mixtures) is at the closest position to this Negligible changes of the asteroid spectra with rotation are indica- temperature range. However, the point of CO2 is shifted downward by 50° (e.g., Longhi, 2005). Therefore, one could expect that it is tions of a relatively homogenous content of the surface matter. Based on the overall shape of the spectra with a minor positive gra- H2O that predominantly survives under the surface of primitive- type asteroids since their formation or since previous falls of smal- dient, 330825 may be ascribed to taxonomic type S or C as follows ler ice bodies on the asteroids (e.g., Fanale and Salvail, 1989; Jewitt from spectral features of other asteroids (Tholen and Barucci, and Guilbert-Lepoutre, 2012). Moreover, parent bodies of C and close type asteroids should include a considerable ice component because of heliocentric zoning of the asteroid belt by 26Al heating (Grimm and McSween, 1993). In turn, intensive sublimation of fresh H2O ice excavated on a primitive asteroid at recent impact (s) may create a coma of icy particles around the body at the highest surface temperatures near its shortest heliocentric distances. To verify our assumption, we estimate the subsolar temperatures on the observed asteroids. As is known, most celestial objects (including planets) have approximately blackbody spectra and effective temperatures described by the Stefan–Boltzmann law. In addition, we relate the received solar electro-magnetic energy by the unit area at an arbitrary top level of terrestrial atmosphere at the subsolar point (that is the solar constant, Ce = Lsun/ 2 2 4pre = 1367 W m , in which Lsun is the luminosity of the Sun, re is Earth’s heliocentric distance) and the radiative energy received Fig. 8. Reflectance spectra of Asteroid 330825 (2008 XE3) normalized at 0.55 lm, by the unit area normal to the direction to the Sun at heliocentric presented in the chronological order, and shifted arbitrarily for clarity. Spectra 1 p 2 p 2 distance r (in AU): E = Ce 4 re /4 r . Accordingly, we and 2 are the averages of three and two similar in shape consecutive spectra, calculate the effective temperature in the subsolar point of an respectively. V.V. Busarev et al. / Icarus 262 (2015) 44–57 51

1989; Bus and Binzel, 2002a, 2002b). Moreover, absence of typical reach about 400 K, whereas many of the known NEAs have perihe- for S-type asteroids olivine–pyroxene band at 0.9 lm in its reflec- lion distances below 0.3 AU where the surface temperature can rise tance spectrum makes 330825 similar to a C-type body. However, above 800 K (Delbo’ et al., 2009). Thus, the sublimation rate of absence of 0.9-lm band could be result of existence of some dark- absorbed or buried near the surface water ice, if present, should ening components (e.g., carbonaceous compounds or magnetite) in considerably increase while the bodies are approaching the Sun. the surface matter of 330825. Thus, we are unable unambiguously Existence of the surface interstitial ice or bound water on different to determine type of 330825 and, hence, its predominant mineral- type asteroids was proved by observations of its characteristic ogy from our data, and state only that it may be C(S). Obviously, it absorption band at 3.0 lm(Lebofsky et al., 1981; Jones et al., should be additionally investigated. 1990; Rivkin et al., 1995, 2000). Also, as shown by modeling and Observational spectra of 2012 QG42 were registered for 2h, calculations (e.g., Zolensky et al., 1989; Shock and Schulte, 1990), which corresponds to only 0.09 portion of its rotation period. synthesis of water-soluble organic compounds was possible in Asteroid reflectance spectra normalized to 1 at 0.55 lm are shown the process of aqueous alteration of insoluble organics in the car- in chronological order in Fig. 9. The spectra are arbitrarily displaced bonaceous meteorite parent bodies. Thus, the main result of con- for clarity of the plot. Spectrum 1 of 2012 QG42 is an average of sidered process of water ice sublimation on the asteroid surface three similar in shape consecutive reflectance spectra. Spectrum should be creation of dark or insufficiently transparent organic 2 is a single reflectance spectrum. It was separated from the aver- films and/or blebs on the surface silicate particles. In turn, the dark aged subset of the first three spectra (Table 1) to show their simi- films must preclude the formation of diffuse component in the larity in common. A maximum in the spectra of 2012 QG42 at reflected light containing information on the surface matter con- 0.50 lm(Fig. 9) and possible extended short-wavelength wing of tent. On the other hand, the presence of relatively transparent fro- an absorption band at 0.9–1.0 lm makes them similar to spectra zen volatiles in between the surface particles on the main-belt of either S-type asteroids or B-type ones having a negative gradient asteroids could make absorption bands in their reflectance spectra of reflectance spectra in the visible to near-IR range (Tholen and more strong. Another important effect of considerable temperature Barucci, 1989; Bus and Binzel, 2002b). Thus, we assume that it fluctuations on the surface of NEAs is mineral decomposition into may be of S or B-type. The surface matter of 2012 QG42 could silicate particles themselves followed by the disappearance of their include not only high-temperature minerals (for instance, pyrox- spectral features. Then, it could explain the absence of weaker ene, olivine, etc.) but also hydrated silicates. To distinguish exactly absorption features and even some changing the overall shape of the type of 2012 QG42, it needs additional investigation of the reflectance spectra of near-Earth asteroids. For these reasons, tax- asteroid in a wider spectral range or by other methods. At the same onomic classification of the bodies is complicated. time, previous publications devoted to (330825) 2008 XE3 and 2012 QG42 stated explicitly that both bodies are of S-type 4. Laboratory investigations of analog samples (Warner et al., 2013; Taradii et al., 2013). An important point should be discussed here. As seen from We performed a laboratory study of some proper analog sam- Figs. 8 and 9, reflectance spectra of both NEAs, (330825) 2008 ples to facilitate interpretation of the obtained reflectance spectra XE3 and 2012 QG42, are featureless compared to the considered of asteroids. Taking into account taxonomic S-type of (32) Pomona main belt asteroids. This suggests that the lack of subtle absorption and its supposed pyroxene–olivine content confirmed by our mea- features in reflectance spectra of near-Earth asteroids may be surements, we rely on previous results of other authors. Actually, results of common environmental or space weathering factors. reflectance spectra of pyroxenes, olivines and their mixtures are The most significant of them are higher surface temperatures and well documented (e.g., Adams, 1974; Singer, 1981; Cloutis et al., more strong darkening action of the solar wind and UV-radiation. 1986; Khomenko and Platonov, 1987; Burns, 1993). On the other As experimental modeling showed, the consequences of the latter hand, it should be recognized that knowledge of spectral character- are surface darkening and reddening of silicate materials from istics of analog samples for primitive type asteroids remains insuf- near-UV to near-IR ranges (e.g., Loeffler et al., 2009; Kanuchová ficient. Investigations of the asteroids themselves and the most et al., 2010). Formation of H2O film of amorphous ice is possible primitive meteorites, as their possible fragments, showed that best directly from the vapor phase onto the surface particles of an aster- spectral analogs of the bodies are carbonaceous chondrites, oid at temperatures below 130 K (Hobbs, 1974). On the contrary, at phyllosilicates (serpentines, chlorites, etc.), oxides and hydroxides 1 AU from the Sun, typical temperatures at the subsolar point (e.g., Gaffey et al., 1989; Calvin and King, 1997; Hiroi and Zolensky, 1999; Cloutis et al., 2011a, 2011b). Carbonaceous chondrites are rare for their instability at falls in terrestrial atmosphere (shocking and heating) and subsequent rapid weathering (e.g., Dodd, 1981). For these reasons, such type of matter is incompletely represented in meteoritic collections. Taking also into account known taxonomic types of (145) Adeona, (704) Interamnia, and (779) Nina as primitive or close to that, we have decided to spare a special attention to CI–CM carbonaceous chondrites and serpentines as a predominant component of the meteorites and possibly surface matter of the asteroids. Thus, we performed a study of some useful CI–CM carbonaceous chondrite and phyllosilicate analog samples (Table 3).

4.1. Carbonaceous chondrites

We undertook reflectance measurements of accessible samples Fig. 9. Reflectance spectra of Asteroid 2012 QG42 normalized at 0.55 lm, presented of carbonaceous chondrites. Four samples of known carbonaceous in the chronological order, and shifted arbitrarily for clarity. Spectrum 1 is the average of three similar in shape consecutive reflectance spectra, respectively. chondrites, Orguel (CI), Mighei (CM2), Murchison (CM2), and Reflectance spectrum 2 is obtained from a single observed spectrum of the asteroid. Boriskino (CM2), were taken from the meteoritic collection of 52 V.V. Busarev et al. / Icarus 262 (2015) 44–57

Table 3 Description of the used serpentine samples.

Sample Place of origin The main constituents (in order of abundance) 1a The North Urals, Russia Chrysotile and lizardite 4a Pobuzh’e, Ukraine Lizardite, chrysotile and calcite 4b Pobuzh’e, Ukraine Lizardite, chrysotile and calcite 10a Bazhenovskoe deposit, Chrysotile and lizardite Urals, Russia 28 Bazhenovskoe deposit, Lizardite and chrysotile Urals, Russia 30 Idzhim deposit, Sayans, Brucite, chrysotile and lizardite Russia 2540 Nizin-Kol river, Kuban’, Russia Chrysotile and lizardite

Fig. 11. Normalized at 0.55 lm and shifted reflectance spectra of powdered Vernadsky Institute of Geochemistry and Analytical Chemistry terrestrial serpentines (numbers 1a, 4a, 4b, 10a, 28, 30, and 2540) (particle sizes (RAS). Reflectance spectra of the ground meteoritic samples 60.15 mm). The spectra are disposed from top to bottom in the order of intensity l (60.25 mm grain size) were measured in the 0.35–1.00 lm range increase of an absorption band at 0.40–0.48 m. with a single-beam spectrophotometer based on a SpectraPro- 275 triple-grating monochromator controlled by a PC. The angles Zolensky, 1999). We noted that the most prominent one centered of incident and reflected light beam and its diameter were 45°, at 0.435–0.445 lm has different relative intensity (up to 25%) 0°, and 5 mm, respectively. Compressed powder of MgO was used in the spectra (Table 2). Excluding our publication (Busarev et al., as a reflectance standard. The root mean square relative error for 2004), a considerable intensity of the absorption band in reflec- the reflectance spectra does not exceed 0.5–1.0% in the visible tance spectra of serpentine samples was not previously reported. and increases gradually to 1–2% at the red end of the operational There is the only published information on detection of an intense 3+ spectral range (Busarev and Taran, 2002). absorption band at 0.44 lminFe -content (Fe2O3 0.5 wt.%) yel- As seen from the measured normalized reflectance spectra, car- low corundum (a Al2O3)(Taran et al., 1993). bonaceous chondrites of CI and CM groups (Fig. 10), have most Microprobe and Mössbauer investigations of the low-iron Mg prominent absorption bands at 0.45–0.57 and 0.80 or 0.75 and serpentine samples were undertaken, as well. Electron microprobe 0.90 lm(Table 2). The spectral features of CI–CM carbonaceous analysis of thin or polished sections of the samples was performed chondrites are in agreement with those found in previous works to determine their elemental composition. The measurements (e.g., Calvin and King, 1997; Cloutis et al., 2011a, 2011b). were made at the scanning electron microscope CamScan-4DV. The accuracy of the measurements is dependant of the content of 4.2. Serpentines constituents: if (C/C) > 10 wt.%, it is 2%, if (C/C) from 5 to 10 wt.%, it is 5%, and if (C/C) from 1 to 5 wt.%, it is 10%. However, some Seven low-iron Mg serpentine samples were also taken as pos- remarks relating the method should be made. As far as the micro- sible analogs for primitive type asteroids. It should be noted that probe is not sensitive to bound water and other volatiles in the such a type of serpentine is typical for an early stage of serpen- layer silicates, the samples were tested for the loss of volatiles by tinization (e.g., Deer et al., 1963; Barber, 1981; Brearley, 2006). igniting. The method of volatile content measurement in a The serpentine samples were picked out from original ones found hydrated sample is based on burning of the sample to a constant in different geological formations in Russia and Ukraine (Table 3). (invariable) mass. The quantity of lost volatiles corresponds to Reflectance spectra of the ground serpentine samples (particle the difference in mass of the sample before and after the annealing. ° sizes 60.15 mm) were measured with the above described Necessary fractions of the samples heated up to 950 C have been single-beam spectrophotometer. The obtained reflectance spectra investigated by the method. With the exception for numbers 4a of the samples are shown in Fig. 11. There are absorption bands and 4b which quantities were too small to get reliable data (calcu- at 0.40–0.50 and 0.80 or 0.75 and 0.90 lm in the spectra lated theoretically) volatile losses were determined for all samples. (Fig. 11). Positions of the features agree with those found in earlier Then preliminary results of the microprobe analysis for the serpen- works (e.g., King and Clark, 1989; Calvin and King, 1997; Hiroi and tine samples were recalculated and given in Table 4. Actually,

Table 4 Elemental analyses for serpentine samples.

Oxides (wt.%) Sample 1a 4a 4b 10a 28 30b 2540

FeO/Fe2O3 2.11 1.35 1.80 1.91 2.62 2.19 2.36 MgO 41.98 38.56 51.61 38.43 38.50 34.65 38.17

Al2O3 0.67 – – 0.76 1.20 0.08 1.25

SiO2 41.33 39.75 36.77 43.72 43.30 39.51 42.50

Cr2O3 – – 0.06 0.60 1.07 0.11 1.34 CaO 0.02 0.06 0.04 0.09 0.01 0.06 0.02

K2O 0.01 0.01 – – – 0.02 0.08

Na2O – 0.06 – – 0.17 0.17 0.03 MnO 0.14 – 0.01 0.08 0.07 0.01 –

P2O5 0.11 0.11 – 0.02 0.07 0.01 0.17

TiO2 0.04 – – – – – – Fig. 10. Normalized at 0.55 lm and shifted reflectance spectra of powders (particle NiO––––––– 6 sizes 0.25 mm) of carbonaceous chondrites, Orguel (CI), Mighei (CM2), Murchison Sum 86.40 79.90 79.9 85.60 87.00 76.80 85.90 (CM2), and Boriskino (CM2) (particle sizes 60.25 mm), taken from the meteoritic collection of Vernadsky Institute of Geochemistry and Analytical Chemistry of RAS. Remarks: The sum of oxides is calculated excluding volatile loses. V.V. Busarev et al. / Icarus 262 (2015) 44–57 53 microprobe measurements show that the serpentine samples are Table 6 magnesian and low-iron ones. Results of the correlation analysis. 2+ 3+ 3+ 3+ Mössbauer spectroscopy of the samples was performed to Sample W044 Fe FeOh FeTh FeOh FeOh+Th obtain data about their Fe2+ and Fe3+ contents and coordinations (nm) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) of the ions. The measurements were carried out at a room temper- 30 0.80 1.7014 1.5925 0.1089 0 0.1089 ature using the MS-1104E spectrometer in constant acceleration 4b 2.30 1.3974 0.7197 0.4625 0.2138 0.6763 mode in absorption geometry. The Co57(Rh) with activity of about 4a 3.70 1.0496 0.4114 0.3464 0.2928 0.6392 10a 4.00 1.4838 0.3561 0.0816 0.3903 0.4719 10 mCi was used as a c-source. Calibration of the spectrometer was a 2540 5.80 1.8362 0.0257 0.1340 0.3874 0.5214 performed by the -Fe standard technique. Processing and analysis 1a 11.09 1.6387 0.3720 0.4916 0.7751 1.2667 of the experimental data were made using the MSTools program 28 12.70 2.0355 0.0875 0.3969 0.7246 1.1215 package for the model fitting of the spectra and restoration of R 0.5551 0.6540 0.4996 0.9640 0.8931 the spectral hyperfine parameter distribution functions. The deter- k 0.1954 0.0397 0.0656 0.1053 mined relative quantities of Fe2+ and Fe3+ (octahedral and/or tetra- Error 0.0479 0.0100 0.0043 0.0123 hedral) in reference to the values of total iron in the samples are Designations: R – the correlation coefficient for the linear regression y = k ⁄ x, where given in Table 5. An assumption was made (Busarev et al., 2004) x = W044 (the equivalent width of the absorption band of serpentines at 0.44 lm) that the absorption band may be characteristic of the serpentine and y is equal to the quantity of Fe (in wt.%) in corresponding valency and coor- 2+ 3+ samples and deserves a special attention. dination: Fe – total quantity of iron, FeOh – ferrous octahedral iron, FeTh – ferric tetrahedral iron, Fe3+ – ferric octahedral iron, Fe3+ – total ferric iron. We performed also laboratory reflectance measurements of the Oh Oh+Th analog samples. At standard experimental conditions (absence of an external diffused light), there is no a background in an absorp- 3+ and FeOh+Th contents in the low-iron (1–2 wt.%) serpentine samples tion band. For this purpose, a special attention has been given to was performed. Results of that are given in Table 6. the issue every measurement. By determination, an absorption Thus, we have found a middle W044–Fe (total) correlation band arises in a reflectance spectrum of a sample as a difference 2+ (R = 0.555), absence of W044–FeOh correlation (R = 0.654), an inter- between the measured spectral brightness and the nearest contin- 3+ mediate W044–FeTh correlation (R = 0.500), and high correlations of uum. Continuum of an absorption band in a reflectance spectrum 3+ 3+ W044–FeOh (R = 0.964) and W044–FeOh+Th (R = 0.893) in the low-iron was removed by its normalization to the continuum of the spec- Mg serpentine samples. The last two linear-proportional depen- trum after removal of the continuum slope. The relative intensity dences can be expressed as the next: of an absorption band or its equivalent width was determined by ð 3þÞð :%Þ¼ : ð Þ this method. Then, the equivalent width (or an area under the N FeOh wt 0 066 W 044 3 drawn arbitrary continuum) of the band was calculated for the normalized reflectance spectrum of each sample according to for- 3þ NðFe þ Þðwt:%Þ¼0:105 W044 ð4Þ mula (2): Oh Th Eq. (4) can be considered as a more universal one as obtained XN for Fe3+ in both coordinations. We suppose that the relation may W044 ¼ ð1 rðkiÞÞDk ð2Þ i¼1 be true not only for low-iron Mg serpentines but also for other low-iron Fe3+ compounds. It follows from theoretical and experi- Dk k W044 is the equivalent width, is the spectral step, r( i) are the mental investigations of Fe3+-content oxides and hydrated oxides residual intensities in the normalized spectrum, and N is the num- (Sherman, 1985; Sherman and Waite, 1985; Bakhtin, 1985). ber of points in the band. Thus, corrected absorption bands at 0.40– However, the assumption should be verified on a larger number l 0.48 m for 28, 1a, 2540, 10a, 4b and 4a serpentine samples are of samples of low-iron Fe3+ compounds. shown in Fig. 6. The values of the relative intensity and other At the same time, we did not find any noticeable absorption parameters of the features are listed in Table 2. We noted that val- band at 0.40–0.48 lm in reflectance spectra of intermediate-iron l ues of the equivalent width of an absorption band at 0.40–0.48 m (5–8 wt.%) lizardite–chrysotile and antigorite samples. As it is in the reflectance spectra of considered serpentine samples form a also known from studies of serpentines and other phyllosilicates common row. The values were disposed in the order of increase with elevated content of iron, there are only minor absorption fea- and compared with values of iron content in the corresponding tures in their reflectance spectra in the range (Calvin and King, 2+ 3+ 3+ samples. We determined quantities of Fe, FeOh,FeTh,FeOh,and 1997; Hiroi and Zolensky, 1999). Based on the results, we conclude 3+ FeOh+Th in the samples in weight percents from the microprobe and that the mechanism of light absorption due to electronic Mössbauer data (Tables 4 and 5) and included them in Table 6 for ECP-transitions in exchangeable Fe3+–Fe3+ pairs is partially or com- correlation analysis. Then, linear regression analysis for the depen- pletely blocked in the case of intermediate to high iron contents. l dence between the equivalent width of 0.44- m absorption band For these reasons, Eq. (4) cannot be used for a quantitative estima- 2+ 3+ 3+ (W044) in the reflectance spectra and Fe (total), FeOh,FeTh,FeOh, tion of Fe3+ content at remote investigations of solid celestial bodies.

Table 5 Absorption bands of (145) Adeona, (704) Interamnia, (779) The relative intensity of the Mössbauer partial spectra. Nina, and the serpentine samples centered at 0.44–0.46 lm are compared in Fig. 6. They may be considered as the same absorption Sample Paramagnetic partial spectra Magnetite band which position is slightly changing because of structural and (Fe3O4) I(Fe3+), % I(Fe2+), % I(Fe3+), % coordination differences in the crystal structure of the matter. Noteworthy, the band of asteroids is lower in relative intensity Td Oh Oh Td Td, Oh than that of serpentines. It may be an indication of an admixture 1a 30.0(1.9) 47.3(2.4) 22.7(1.4) – – of either different matter or similar one but with higher iron con- 4a 33.0(6.3) 27.9(6.9) 39.2(1.6) – – 4b 33.1(4.0) 15.3(3.0) 51.5(2.8) – – tent. At the same time, such a band is absent in reﬂectance spectra 10a 4.8(1.4) 36.9(4.6) 25.2(2.9) – 21.2(2.8) of carbonaceous chondrites (Fig. 10). There are also two subtle 28 15.2(2.2) 24.1(3.3) 3.2(0.5) – 40.9(3.3) absorption features in the range 0.36–0.40 lm in reﬂectance spec- 2540 6.5(1.0) 28.9(2.9) 2.2(0.3) – 50.6(2.6) tra of some serpentine samples (Fig. 11, Table 2) coinciding with 30 6.4(0.3) – 93.6(0.9) – – that of the asteroids (Fig. 7). Therefore, we suggest that the surface 54 V.V. Busarev et al. / Icarus 262 (2015) 44–57 matter of Adeona, Interamnia, and Nina is a mixture of carbonaceous chondrites, hydrated silicates of serpentine type and Fe3+- content oxides and/or hydroxides.

5. Discussion

S-type of Pomona is confirmed by the results of our observations. We registered also weak absorption bands at 0.49–0.55 and 0.73–0.77 lm, which may be explained by the presence of the hetero-valent Fe2+ and Fe3+ ions in Pomona’s surface matter. The spectral features could originate from electronic d–d- transitions and are strengthened by ECP-transitions between pairs of Fe3+-cations and/or at Fe2+ ? Fe3+ IVCT-transitions. The absorption bands slightly changing with asteroid rotation may point out Fig. 12. Absorption G-band in spectra of some F-G-stars including HD 10307 used in the work as a solar analog. to variations in the relative contents of the Fe2+ and Fe3+ ions and, therefore, in the oxidation state of Pomona’s surface matter. Spectral similarity of the obtained reflectance spectra of (145) Adeona, (704) Interamnia, and (779) Nina of close taxonomic types hydrated silicates of serpentine type and Fe3+-content oxides is expressed in the presence of nearly coinciding absorption fea- and/or hydroxides. This implies similar physical–chemical condi- tures at 0.36–0.39, 0.40–0.47, and 0.50–0.85 lm. The first two of tions of origin and/or evolution of the bodies. them are likely spectral features of alone ferric iron. The most It should be also considered a possibility of influence of G-band prominent common spectral feature of the asteroids and their ana- of F-G-stars used as solar analogs (e.g., Hardorp, 1980) on the Fe3+- logs, CI–CM carbonaceous chondrites and serpentines, is a wide band in reflectance spectra of asteroids. As seen in Fig. 12, spectral absorption band at 0.55–1.00 lm(Figs. 2–4, 10 and 11). It origi- position of the G-band (centered at 0.43 lm) overlaps partly with nates possibly due to Fe2+ ? Fe3+ IVCT-transitions (e.g., Platonov, the Fe3+-band. A spectrum of our solar analog (HD10307) is shown 1976; Bakhtin, 1985; Burns, 1993). The spectral similarity of car- in Fig. 12, as well. Noteworthy, its spectral type is very close to that bonaceous chondrites and phyllosilicates (including serpentines) of the Sun (G2V). Taking into account the used methodology of is explained by predominance of the latter (up to 90 wt.%) in asteroid reflectance spectrum calculation, we conclude that the the matrix content of the first (e.g., Dodd, 1981). Reflectance spec- influence of G-band is possible in the case of spectral differences tra of CI–CM carbonaceous chondrites in the range 0.3–2.5 lm between a solar analog and the Sun itself. To avoid this we need extensively recently studied by Cloutis et al. (2011a, 2011b).Itis to choose solar analogs more carefully according to their spectral important to note that the measurements were performed at dif- characteristics. ferent phase angles. The main conclusion of the works about spec- From our spectrophotometric observations and spectral data tral similarity of carbonaceous chondrites and phyllosilicates is in from the SMASSII survey and asteroid taxonomy (Bus and Binzel, accordance with ours. 2002a, 2002b), 330825 and 2012 QG42 types are assessed by us We undertook additional investigations of low-iron Mg serpen- as C or S and S or B, respectively. At the same time, previous pho- tine samples with an absorption band at 0.40–0.48 lm. Micro- tometric and spectral observations of 330825 and 2012 QG42 probe and Mössbauer measurements of the samples showed that showed that both bodies are of S-type (Warner et al., 2013; there is a strong correlation between the equivalent width of the Taradii et al., 2013). From the published results, we conclude that band and Fe3+ content. This confirms likely the proposed mecha- our measurements could reflect some surface heterogeneity of the nism of the band origin for electronic ECP-transitions in exchange- asteroids, especially for 2012 QG42. At the same time, reflectance able Fe3+–Fe3+ pairs (Sherman, 1985; Sherman and Waite, 1985). spectra of the both NEAs are featureless compared to the consid- We found that the highest intensity of the band is reached only ered main belt asteroids. Among possible reasons, we need to in reflectance spectra of low-iron (no more then 3 wt.% of FeO) emphasize two common. It may be due to action of weathering fac- Mg serpentines. At the same time, Fe3+ absorption band at 0.40– tors, such as higher surface temperatures and a stronger darkening 0.48 lm is absent or has a low intensity in reflectance spectra of by the solar wind and UV-radiation. For instance, in the case of more ferrous serpentine samples. We conclude that the electronic presence of frozen and/or adsorbed water ice and volatile organics mechanism of the light absorption is partially or completely in the asteroid surface matter, the approaching of the bodies to the blocked at higher iron contents. Thus, in general, the absorption Sun should lead to increasing sublimation of frozen volatiles and feature can be used only as a qualitative indicator of ferric iron the formation of dark organic films on the regolith particles. At in oxidized and hydrated low-Fe compounds on the surface of the reflection of solar light from the asteroid surface, the dark films asteroids. could preclude the formation of a diffuse component containing However, an absorption band centered at 0.44–0.46 lmin information on the matter composition. reflectance spectra of the asteroids and serpentines is absent in Discovering for the fist time apparent spectral signs of sublima- spectra of carbonaceous chondrites (Figs. 6 and 10). The result is tion activity on (145) Adeona, (704) Interamnia, and (779) Nina confirmed by measurements of other authors (e.g., Hiroi and deserve a special consideration. The asteroids are primitive and Zolensky, 1999; Cloutis et al., 2011a, 2011b). It is probably the low-albedo, though Nina’s geometric albedo is twice higher. It result of a considerably higher ferrous content in the latter. The suggests either existence of a native water ice under the surface values of FeO contents in the matrix of our three carbonaceous preserved since asteroid formation or, on the contrary, buried at chondrite samples, Orguel (CI), Mighei (CM2), and Murchison previous collisional events and recently resurfaced ice (e.g., (CM2) are 22.08, 27.21, and 33.43 wt.%, respectively (Zolensky Fanale and Salvail, 1989; Jewitt and Guilbert-Lepoutre, 2012). et al., 1993). From a lower intensity of Fe3+ band at 0.44–0.46 lm Interamnia and Nina were in time of the observations near the per- in reflectance spectra of studied primitive asteroids then in those ihelion distance to the Sun and their surface temperatures around of Mg serpentine samples, the supposition is made that the surface the subsolar point reached a maximum (238.6 K and 257.3 K, matter of the asteroids is a mixture of carbonaceous chondrites, respectively). Approaching to the Sun Adeona was near a middle V.V. Busarev et al. / Icarus 262 (2015) 44–57 55

Found spectral similarity of the obtained reflectance spectra of (145) Adeona, (704) Interamnia and (779) Nina of close taxonomic types is expressed in the presence of nearly coinciding absorption features at 0.36–0.39, 0.40–0.47, and 0.50–0.85 lm. The absorption bands are predominantly manifestations of the electronic crystal- field d–d-transitions in Fe3+ ions strengthened by transitions in magnetic exchange-coupled pairs (ECP) of neighboring cations, Fe2+–Fe3+ and/or Fe2+–Fe3+ (Rossman, 1975; Sherman, 1985; Sherman and Waite, 1985). The most prominent common spectral feature of the Asteroids Adeona, Interamnia, and Nina and their analogs, such as CI–CM carbonaceous chondrites and serpentines, is a wide absorption band at 0.55–1.00 lm(Figs. 2–4, 10 and 11). It is probably due to Fe2+ ? Fe3+ IVCT-transitions (e.g., Platonov, 1976; Bakhtin, 1985; Burns, 1993). Thus, the carbonaceous chondrites and serpentines may be considered as the best Fig. 13. Reproduced with permission after insignificant changes figure with model spectral analogs for the studied primitive asteroids. A considerable reflectance spectra of an asteroid covered with 10-lm particles of water ice and tholins mixture (‘‘asteroid”), a spherical coma consisting of 0.2-lm particles of the fraction of the observed asteroids could be covered with these same material (2e+16 m1 particle concentration) surrounding the asteroid materials. This implies similar physical–chemical conditions of (‘‘coma”), and the combined system (‘‘total”) (Carvano and Lorenz-Martins, 2009). the origin and/or evolution of the asteroids. We investigated more thoroughly seven samples of low-iron Mg serpentine having an absorption band at 0.40–0.48 lm which heliocentric distance but its subsolar temperature turned out to be is similar to that of Adeona, Interamnia, and Nina. Microprobe very close to that of Interamnia. We calculated also heliocentric and Mössbauer measurements of the samples showed that the distances of Adeona, Interamnia, and Nina on moments of its spec- equivalent width of the band has a strong correlation with Fe3+ tral observations by other authors. Adeona’s heliocentric distance content. The relative intensity of the band is the highest in reflec- was considerably less only at observations made by McCord and tance spectra of low-iron (no more than 3 wt.% of FeO) serpentines. Chapman (1975a) then that in ours, but we see no indications of In addition, we found experimental evidence that the absorption discussed features in their data (Fig. 2a). Published reflectance feature is weak or absent in reflectance spectra of serpentine sam- spectra of Interamnia (Bus and Binzel, 2002a; Lazzaro et al., ples having higher iron content. This means that the electronic 2004; Fornasier et al., 2014) were obtained close to its aphelion mechanism of the band is partially or entirely blocked in the case. distance to the Sun. Finally, published spectral data on Nina (Bus Therefore, the absorption feature can be used only as a qualitative and Binzel, 2002a; Lazzaro et al., 2004) were measured around indicator of ferric iron on the surface of asteroids and other atmo- its middle heliocentric distance to the Sun. We conclude that sphereless celestial bodies. It is important to note that the band of reflectance spectra of discussed asteroids at perihelion distances considered asteroids is lower in relative intensity than that of ser- were not previously obtained or just rejected because of their dis- pentines (Fig. 6). It may point to presence in the asteroid surface similarity with preceding ones. For example, Bus and Binzel matter either an admixture of spectrally different compounds or (2002a) used a method of real-time examination for each regis- similar ones but with higher iron content. tered asteroid spectrum (quick-look reduction and dividing by We observed Asteroids (704) Interamnia, and (779) Nina near the spectrum of analog star) to estimate its quality. perihelion heliocentric distances and (145) Adeona about its mid- To explain unusual reflectance spectra of some distant asteroids dle heliocentric distance and found for the first time spectral signs by the process of ice sublimation, Carvano and Lorenz-Martins of a comet activity on their surface. It is manifested likely in the (2009) for a model of a spherical asteroid reflecting light according form of predominantly H2O ice (buried previously and resurfaced Hapke’s formula and surrounded by a faint coma of ice particles by recent impacts) sublimation and subsequent scattering by sub-

(consisting of a mixture of H2O and tholins) calculated combined micron ice particles of reflected sunlight in the short-wavelength reflectance spectra of such model system at different concentra- range. It seems such scattering does not change position of intrin- tions of 0.2-lm particles in the coma. We show one of their spectra sic asteroid absorption bands but exaggerates their intensity. corresponding to a slightly elevated dust density (2e+16 m1 con- Taxonomic types of 330825 and 2012 QG42 are assessed as C(S) centration particles) in the coma (Fig. 13). One can see that shape and S(B), respectively. This is only in partial agreement with previ- of the spectrum in the range 0.4–0.6 lm is very close to those of ous photometric and spectral observations of 330825 and 2012 Adeona, Interamnia, and Nina (Figs. 2–4). We consider this a strong QG42 showed that both bodies are of S-type (Warner et al., background for discovering their sublimation activity. Obviously, 2013; Taradii et al., 2013). To specify mineralogy of the asteroids, the asteroids should be more thoroughly investigated in the near their additional investigation in a wider spectral range is desirable. future. Due to differences in geometric observational parameters, our measurements could characterize some surface heterogeneity of the asteroids, especially for 2012 QG42 as a slower rotator. 6. Summary and conclusions However, it should be noted that reflectance spectra of the NEA asteroids are featureless compared to the main-belt asteroids We obtained and analyzed reflectance spectra of different aster- (see Figs. 2–4, 8 and 9). Among possible factors, which could be oids, namely: S-type (32) Pomona with predominantly high- responsible for the effect, we suggest two of them of most rele- temperature mineralogy, (145) Adeona, (704) Interamnia, and vance and importance, namely higher surface temperatures and a (779) Nina with predominantly low-temperature mineralogy, more intensive darkening the surface matter by the solar wind (330825) 2008 XE3 and 2012 QG42 which mineralogy should be and UV-radiation at lower heliocentric distances. In particular, specified. S-type of Pomona is confirmed by the results of our dark organic films could be formed on the regolith particles on observations. At the same time, we registered weak absorption the surface of asteroids while approaching the Sun to about 1 AU bands at 0.49–0.55 and 0.73–0.77 lm explained by the presence or less. Indeed, the solar heating is able to substantially increase of the hetero-valent Fe2+ and Fe3+ ions in Pomona’s surface matter. sublimation of buried or adsorbed ices and volatile organics, if 56 V.V. Busarev et al. / Icarus 262 (2015) 44–57 any. The resulting dark organic films on silicate particles must pre- Bus, S.J., Binzel, R.P., 2002a. Phase II of the small main-belt asteroid spectroscopic clude formation of diffuse component in the reflected light trans- survey. The Observations. Icarus 158, 106–145. Bus, S.J., Binzel, R.P., 2002b. Phase II of the small main-belt asteroid spectroscopic mitted through the asteroid surface matter. If so, it complicates survey. A Feature-Based Taxonomy. Icarus 158, 146–177. NEAs’ taxonomic classification. Bus, S., Binzel, R.P., 2003a. 32 Pomona CCD Spectrum. EAR-A-I0028-4-SBN0001/ Probably, the most interesting results of our work are discov- SMASSII-V1.0:32_01_TAB. NASA Planetary Data System. Bus, S., Binzel, R.P., 2003b. 145 Adeona CCD Spectrum. 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